No, I did not. Thanks for the hint. When I do connect data and ground, the setup works as expected. So there seems to be something missing with the RF modules (I also already tried different modules, with the same result).

Those spring antennae are utter garbage. I use solid core wire from ethernet cable for antennae.

I would also try swapping in different transmitter and receiver modules. Yours *looks* like a good receiver (is that a SYN470 IC on there? Those work very well); I didn't try any transmitters with an IC in SOIC-8 package when I was testing these, so can't comment on that.

How far apart are the modules?If you have them really close together like 10 cms or less the transmitter can overload the receiver causingerratic operation.The helical antennas arnt as good as 17cm antennas, but they wont stop the modules working.The other possibility is that there is an interfering source nearby.